JWST NIRCam Imaging of NGC 4258: I. Observation Overview

TL;DR

This study uses JWST NIRCam imaging of NGC 4258 to investigate AGN-driven shocks and their effects on the interstellar medium, revealing low-velocity shocks and dust grain destruction in the galaxy's disk.

Contribution

First detailed near-infrared imaging of NGC 4258's shock regions with parsec-scale resolution, linking shock properties to AGN feedback mechanisms.

Findings

Shocks in bright radio regions are low-velocity (50-100 km/s).

Multi-phase shocks are indicated by co-spatial [Fe II] and H$_2$ emission.

PAH emission is weaker or absent in shock-excited regions.

Abstract

We present James Webb Space Telescope (JWST) NIRCam imaging of the nearby Seyfert 1.9 galaxy NGC 4258, which hosts strong star formation regions as well as an anomalous jet-like radio structure that extends through a significant portion of its disk. This galaxy provides a unique environment to study Active Galactic Nucleus (AGN)-driven shocks and their impact on the interstellar medium (ISM) as its proximity allows for narrow-band observations of various near-infrared tracers sensitive to multiple levels of shock and radiative excitation: [Fe II] (1.64 $\mu$m), Pa$\alpha$ (1.87 $\mu$m), H$_2$ (2.21 $\mu$m), 3.3 $\mu$m polycyclic aromatic hydrocarbon (PAH) emission, Br$\alpha$ (4.05 $\mu$m), and Pf$\beta$ (4.66 $\mu$m), allowing us to trace shocks with parsec-scale resolution. Comparing these near-infrared observations with available ultraviolet, optical, radio, and X-ray imaging, we find that shocks present in the brightest regions of the anomalous radio structure are likely of low-velocity (50-100 km s$^{-1}$), suggesting that these features originate from AGN-driven winds that interact with the host medium and mechanically impart energy into the disk. Further, while co-spatial [Fe II] and H$_2$ emission indicate multi-phase shocks, PAH emission is relatively weaker or absent in the most shock-excited regions, consistent with the destruction of small dust grains. Finally, we propose that surveys identifying enhanced [Fe II] in AGN host galaxies may systematically reveal a key population where AGN feedback is significantly coupled with the surrounding ISM and actively shaping galaxy evolution.